"The ruling departs from preceding judgments at the Supreme Court... and contains serious mistakes in the interpretation of the
law," said Mr Hiranuma.

But people living near the controversial reactor welcomed Monday's decision.

Aileen Mioko Smith, an activist who has been campaigning on the issue for many years, said it was an "epoch-making decision".

She said it was a significant strike at "the whole raison d'etre" of the Japanese nuclear industry.

Nuclear needs

With few natural resources, Japan relies heavily on its 51 nuclear power plants to supply about a third of its electricity.

The government wants to raise that to 42% by the year 2010.

The reactor at Monju is central to that plan.

The government has already spent 780bn yen ($6.56bn) on
the project, including 580bn yen to build the experimental fast-breeder reactor, which is designed to produce more nuclear fuel than it consumes.

But with public fears high since an accident at Tokaimura in 1999, the government faces many hurdles in its plans for nuclear growth.